Excited States of Bridged [14]Annulenes

Fluorescence and Raman spectra of two representative bridged [14]annulenes, i.e., 1,6:8,13-ethanediylidene[14]- annulene and 1,6:8,13-propanediylidene[14]annulene (1 and 2 in Figure 1, respectively) have been measured. The fluorescence spectra show a resolved Franck-Condon structure assigned to low-frequency totally symmetric modes and their overtones/combinations. The Raman spectra have been measured using several excitation wavelengths in pre- and/or near-resonance conditions with respect to the La and Ba states. A1 and a2 vibrational modes are resonantly enhanced in both cases. Excited-state equilibrium geometries and energies of 1 and 2 have been calculated by means of the MCSCF/CAS ab initio procedure. Franck-Condon factors have been determined coupling structural data with normal mode displacements. On this basis fluorescence and Raman excitation profiles are reasonably well fitted to the experiment. It is concluded that the Lb, La, and Ba states of 1 and 2 have equilibrium structures accurately described by the present calculations.